1. Field of the Invention
The present invention relates to systems for controlling the power applied to a load and more particularly is disclosed as it would be adapted to control appliances and the like such as a microwave oven.
Generally speaking, the control circuit of the present invention includes a triac means for controlling alternating current power and means for controlling the triac means.
2. Description of the Prior Art
Many appliance control systems exist which allow the user to vary the power level applied to a function of the appliance and/or the temperature to which a material is to be heated or cooled by the appliance; however, most of these control systems utilize a timing mechanism to determine the states of the control system at various time intervals. The utilization of time as a means for determing when a certain function of an appliance should operate and when that function should stop and another function begin in most instances wastes energy and leads to performance uncertainties. Furthermore, it may be desirable in some appliances to heat or cool a material load at a very slow rate until it reaches a desired temperature. An example of an appliance where the use of these types of control systems is particularly advantageous is a microwave oven.
Specific examples of prior art control systems used in microwave ovens for varying the power level applied to the oven magnetron are disclosed in the following U.S. Pat. Nos. 4,012,617; and 4,023,004.
In those prior art control systems which allow the user of a microwave oven to select a desired power level to be applied to the oven magnetron, the variable power source usually includes an oscillator circuit having a variable period which is a function of the means for selecting the desired power level, and in conjunction with the variable power source a means for detecting the peak of the A.C. line voltage is usually provided to assure that power is applied to the magnetron high voltage transformer at the peak of the A.C. voltage. Most of these prior art variable power sources apply power to the mangetron at the level desired and completely deactivate the magnetron either when a time period has expired or when a desired temperature is reached. However, in a microwave oven it is many times desirable after the food has been cooked that the oven keep the food warm until the user can either remove the food or otherwise reprogram the oven. Furthermore, most prior art peak detector circuits are very sensitive to component values of the circuit and/or require complex logic circuitry to control the on and off switching of the magnetron because of their inability to completely assure that all enabling signals will be provided at voltage peaks.
Most control systems of the type described above when adapted for use in a microwave oven further include circuitry to drive the oven magnetron in response to the variable power source. These drive circuits usually include at least one TRIAC and means to control the supply of gate current to activate and deactivate the TRIAC. Typically these prior art TRIAC drive circuits utilize a switch placed in series with the TRIAC gate to regulate the flow of gate current. Accordingly, these switches must support the entire voltage of the gate current power supply when open or off requiring the use of more expensive switches.
As a result of the disadvantages of prior art variable power temperature control systems described above, such control systems are expensive and many times not as reliable as they could be. By improving upon the design of these older systems the cost can be reduced while at the same time increasing reliability and performance.